<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>demo</title>
</head>

<body>
    <my-text></my-text>

    <script>
        class MyText extends HTMLElement {
            constructor() {
                super();

            }
            connectedCallback() {
                this.append("我的文本");
            }
        }
        window.customElements.define("my-text", MyText);
    </script>
    <div></div>
    <my-text></my-text>
</body>

</html>